Интерфейсный документ GPS, страница 8

The users’convolutional decoders will introduce a fixed delay that depends on their respective algorithms (usually 5 constraintlengths, or 35 bits), for which they must compensate to determine system time from the received signal. Thisconvolutional decoding delay and the various relationships with the start of the data block transmission and SV timeare illustrated in Figure 3-15.IS-GPS-200D7 Dec 200438G2 (133 OCTAL)OUTPUT SYMBOLS(50 SPS)DATA INPUT(25 BPS)(ALTERNATING G1/G2)G1 (171 OCTAL)Figure 3-14.SYMBOLCLOCKConvolutional EncoderENCODED DATA BLOCKTRANSMITTED ON L2DATA BLOCKDECODED BYUSERENCODEDDATA BLOCKRECEIVEDBY USERUSER’S DECODING DELAYDOWNLINK DELAYLATEREARLYSV 12 SECOND EPOCHSFigure 3-15.Convolutional Transmit/Decoding Timing RelationshipsIS-GPS-200D7 Dec 2004393.3.4GPS Time and SV Z-Count.GPS time is established by the Control Segment and is referenced toCoordinated Universal Time (UTC) as maintained by the U.S.

Naval Observatory (UTC(USNO)) zero time-pointdefined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largest unit used in statingGPS time is one week defined as 604,800 seconds. GPS time may differ from UTC because GPS time shall be acontinuous time scale, while UTC is corrected periodically with an integer number of leap seconds. There also is aninherent but bounded drift rate between the UTC and GPS time scales. The OCS shall control the GPS time scale tobe within one microsecond of UTC (modulo one second).The NAV data contains the requisite data for relating GPS time to UTC. The accuracy of this data during thetransmission interval shall be such that it shall relate GPS time (maintained by the MCS of the CS) to UTC(USNO) within 90 nanoseconds (one sigma).

This data is generated by the CS; therefore, the accuracy of thisrelationship may degrade if for some reason the CS is unable to upload data to a SV. At this point, it is assumedthat alternate sources of UTC are no longer available, and the relative accuracy of the GPS/UTC relationship willbe sufficient for users. Range error components (e.g.

SV clock and position) contribute to the GPS time transfererror, and under normal operating circumstances (two frequency time transfers from SV(s) whose navigationmessage indicates a URA of eight meters or less), this corresponds to a 97 nanosecond (one sigma) apparentuncertainty at the SV. Propagation delay errors and receiver equipment biases unique to the user add to this timetransfer uncertainty.IS-GPS-200D7 Dec 200440In each SV the X1 epochs of the P-code offer a convenient unit for precisely counting and communicating time.Time stated in this manner is referred to as Z-count, which is given as a 29-bit binary number consisting of twoparts as follows:a.The binary number represented by the 19 least significant bits of the Z-count is referred to as the time ofweek (TOW) count and is defined as being equal to the number of X1 epochs that have occurred since thetransition from the previous week.

The count is short-cycled such that the range of the TOW-count isfrom 0 to 403,199 X1 epochs (equaling one week) and is reset to zero at the end of each week. The TOWcount's zero state is defined as that X1 epoch which is coincident with the start of the present week. Thisepoch occurs at (approximately) midnight Saturday night-Sunday morning, where midnight is defined as0000 hours on the UTC scale which is nominally referenced to the Greenwich Meridian. Over the years theoccurrence of the "zero state epoch" may differ by a few seconds from 0000 hours on the UTC scale sinceUTC is periodically corrected with leap seconds while the TOW-count is continuous without suchcorrection. To aid rapid ground lock-on to the P-code signal, a truncated version of the TOW-count,consisting of its 17 most significant bits, is contained in the hand-over word (HOW) of the L1 and L2 NAVdata (D(t)) stream; the relationship between the actual TOW-count and its truncated HOW version isillustrated by Figure 3-16.b.The ten most significant bits of the Z-count are a modulo 1024 binary representation of the sequentialnumber assigned to the current GPS week (see paragraph 6.2.4).

The range of this count is from 0 to 1023with its zero state being defined as the GPS week number zero and every integer multiple of 1024 weeks,thereafter (i.e. 0, 1024, 2048, etc.).IS-GPS-200D7 Dec 200441P(Y)-CODE EPOCH(END/START OF WEEK)X1 EPOCHS1.5 sec0403,192403,19612345678403,199DECIMAL EQUIVALENTSOF ACTUAL TOW COUNTSSUBFRAME EPOCHS6 sec100,7990123DECIMAL EQUIVALENTS OF HOW-MESSAGE TOW COUNTSNOTES:1.TO AID IN RAPID GROUND LOCK-ON THE HAND-OVER WORD (HOW ) OF EACHSUBFRAME CONTAINS A TRUNCATED TIME-OF-WEEK (TOW) COUNT2.THE HOW IS THE SECOND WORD IN EACH SUBFRAME (REFERENCEPARAGRAPH 20.3.3.2).3.THE HOW-MESSAGE TOW COUNT CONSISTS OF THE 17 MSBs OF THEACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME.4.TO CONVERT FROM THE HOW-MESSAGE TOW COUNT TO THE ACTUAL TOWCOUNT AT THE START OF THE NEXT SUBFRAME, MULTIPLY BY FOUR.5.THE FIRST SUBFRAME STARTS SYNCHRONOUSLY WITH THE END/START OFWEEK EPOCH.Figure 3-16.

Time Line Relationship of HOW MessageIS-GPS-200D7 Dec 2004424. NOT APPLICABLEIS-GPS-200D7 Dec 200443(This page intentionally left blank.)IS-GPS-200D7 Dec 2004445. NOT APPLICABLEIS-GPS-200D7 Dec 200445(This page intentionally left blank.)IS-GPS-200D7 Dec 2004466. NOTES6.1 AcronymsAI-Availability IndicatorAODO-Age of Data OffsetA-S-Anti-SpoofingAutonav-Autonomous NavigationBPSK-Bi-Phase Shift KeyCDC-Clock Differential CorrectionCNAV-Civil Navigationcps-cycles per secondCRC-Cyclic Redundancy CheckCS-Control SegmentDC-Differential CorrectionDN-Day NumberEAROM-Electrically Alterable Read-Only MemoryECEF-Earth-Centered, Earth-FixedECI-Earth-Centered, InertialEDC-Ephemeris Differential CorrectionEOE-Edge-of-EarthEOL-End of LifeERD-Estimated Range DeviationFEC-Forward Error CorrectionGGTO-GPS/GNSS Time OffsetGNSS-Global Navigation Satellite SystemGPS-Global Positioning SystemHOW-Hand-Over WordICC-Interface Control ContractorID-IdentificationIERS-International Earth Rotation and Reference Systems ServiceIS-GPS-200D7 Dec 200447IODC-Issue of Data, ClockIODE-Issue of Data, EphemerisIRM-IERS Reference MeridianIRP-IERS Reference PoleIS-Interface SpecificationISC-Inter-Signal CorrectionLSB-Least Significant BitLSF-Leap Seconds FutureL2 C-L2 Civil SignalL2 CL-L2 Civil-Long CodeL2 CM-L2 Civil-Moderate CodeMCS-Master Control StationMSB-Most Significant BitNAV-NavigationNDUS-Nudet Detection User SegmentNMCT-Navigation Message Correction TableNSC-Non-Standard C/A-CodeNSCL-Non-Standard L2 CL-CodeNSCM-Non-Standard L2 CM-CodeNSY-Non-Standard Y-codeOBCP-On-Board Computer ProgramOCS-Operational Control SystemPRN-Pseudo-Random NoiseRF-Radio FrequencyRMS-Root Mean SquareSA-Selective AvailabilitySEP-Spherical Error Probablesps-symbols per secondIS-GPS-200D7 Dec 200448SS-Space SegmentSV-Space VehicleSVN-Space Vehicle NumberTBD-To Be DeterminedTBS-To Be SuppliedTLM-TelemetryTOW-Time Of WeekUE-User EquipmentURA-User Range AccuracyURE-User Range ErrorUS-User SegmentUSNO-U.S.

Naval ObservatoryUTC-Coordinated Universal TimeWGS 84-World Geodetic System 1984WN-Week NumberWNe-Extended Week NumberIS-GPS-200D7 Dec 200449(This page intentionally left blank.)IS-GPS-200D7 Dec 2004506.2 Definitions6.2.1 User Range Accuracy. User range accuracy (URA) is a statistical indicator of the ranging accuraciesobtainable with a specific SV. URA is a one-sigma estimate of the user range errors in the navigation data for thetransmitting satellite.

It includes all errors for which the Space and Control Segments are responsible. It does notinclude any errors introduced in the user set or the transmission media. While the URA may vary over a givensubframe fit interval, the URA index (N) reported in the NAV message corresponds to the maximum value of URAanticipated over the fit interval.6.2.2 SV Block Definitions. The following block definitions are given to facilitate discussion regarding thecapability of the various blocks of GPS satellites to support the SV-to-US interface.6.2.2.1 Developmental SVs. The original concept validation satellites developed by Rockwell International anddesignated as satellite vehicle numbers (SVNs) 1-11 are termed "Block I" SVs.

These SVs were designed to provide3-4 days of positioning service without contact from the CS. These SVs transmitted a configuration code of 000(reference paragraph 20.3.3.5.1.4). There are no longer any active Block I SVs in the GPS constellation. The lastBlock I SV was decommissioned in 1995.6.2.2.2 Operational SVs. The operational satellites are designated Block II, Block IIA, Block IIR, Block IIR-M andBlock IIF SVs.

Characteristics of these SVs are provided below. Modes of operation for these SVs and accuracy ofpositioning services provided are described in paragraphs 6.3.2 through 6.3.4. These SVs transmit configurationcodes as specified in paragraph 20.3.3.5.1.4. The navigation signal provides no direct indication of the type of thetransmitting SV.6.2.2.2.1 Block II SVs. The first block of full scale operational SVs developed by Rockwell International aredesignated as SVNs 13-21 and are termed "Block II" SVs. These SVs were designed to provide 14 days ofpositioning service without contact from the CS.6.2.2.2.2 Block IIA SVs.

The second block of full scale operational SVs developed by Rockwell International aredesignated as SVNs 22-40 and are termed "Block IIA" SVs. These SVs are capable of providing 60 days ofpositioning service without contact from the CS.IS-GPS-200D7 Dec 2004516.2.2.2.3 Block IIR SVs. The block of operational replenishment SVs developed by Lockheed Martin aredesignated as SVNs 41-61 and are termed "Block IIR" SVs. These SVs have the capability of storing at least 60days of navigation data with current memory margins, while operating in a IIA mode, to provide positioning servicewithout contact from the CS for that period. (Contractual requirements for these SVs specify transmission of correctdata for only 14 days to support short-term extended operations while in IIA mode.) The IIR SV will provide aminimum of 60 days of positioning service without contact from the CS when operating in autonomous navigation(Autonav) mode.6.2.2.2.4 Block IIR-M SVs.

The subset of operational replenishment SVs developed by Lockheed Martin whichare “Modernized” configuration of “Block IIR” SVs are termed “Block IIR-M”.6.2.2.2.5 Block IIF SVs. The block of operational replenishment SVs developed by Boeing are designated as SVNs62-73 and are termed “Block IIF” SVs. This is the first block of operational SVs that transmit the L5 Civil signal.These SVs will provide at least 60 days of positioning service without contact from the CS.6.2.3 Operational Interval Definitions.